Our invention relates to electronic devices in general and, in particular, to a Schottky semiconductor device and to a method of making such a device. The Schottky semiconductor device of our invention finds typical use as a diode well suited for high voltage applications.
The Schottky barrier diode, or the Schottky diode for short, is a kind of pn junction diode in which the barrier between two regions of opposite conductivity type produces the rectification. Lower power units are usually called semiconductor diodes, and the higher power units are usually called semiconductor rectifiers. The Schottky diode has found extensive usage in high frequency rectifier circuits and similar applications by virtue of its fast switching capability and low power loss. In some instances, however, these advantages have been offset by its susceptibility to surface breakdown. This weakness has generally been attributed to the markedly lower voltage withstanding capability of the Schottky diode at the periphery of the Schottky barrier compared with that at the midportion of the barrier.
We are aware of some conventional solutions to the above problem. Typical of such solutions is the use of either metal field plates, known also as metal overlaps, or guard rings, or a combindation of both. Reference may be had to pages 297-304 of Physics of Semiconductor Devices, second edition, by S. M. Sze for more details on various conventional metal semiconductor device structures.
Let us first study the prior art field plate Schottky diode. It has an n type semiconductor region overlying an n+ type semiconductor region and underlying an electrode of a metal capable of forming a Schottky barrier, this electrode being herein termed "barrier metal electrode". Also formed on the n type semiconductor region is an insulating layer surrounding the barrier metal electrode. The field plate is formed on the insulating layer in electric contact with the barrier metal electrode.
Upon application of a reverse voltage between the barrier metal electrode and an ohmic contact on the bottom of the n+ type semiconductor region, a depletion layer is created in the n type semiconductor region under the barrier metal electrode. Additionally, owing to the field effect of the field plate, another depletion layer is created in the n type semiconductor region in underlying relation to the field plate. The concentration of the electric field under the barrier metal electrode is thus prevented. However, the resulting improvement in the voltage withstanding capability at the periphery of the Schottky barrier has been limited and has not been so satisfactory as could be desired.
Next to be considered is the Schottky diode of the known guard ring design. It has, instead of the field plate, a p+ type semiconductor guard ring formed on the n type semiconductor region so as to surround and contact the barrier metal electrode. The guard ring creates a pn function between itself and the n type semiconductor region. When subjected to a reverse voltage, the pn junction creates a depletion layer thereby improving the voltage withstanding capability of the diode at the periphery of the barrier metal electrode. Essentially, however the guard ring diode can be thought of as a parallel arrangement of Schottky diode and pn junction diode. This arrangement gives rise to the injection of minority carriers at the pn junction when a forward current flows, thereby sacrificing the inherent fast response of the Schottky barrier diode.